1. Field of the Invention
The present invention generally relates to semiconductor devices formed using sidewall image transfer, and, more particularly, to a self-aligned gate sidewall spacer and method of forming the sidewall spacer in a corrugated FET structure, whereby a sidewall spacer is formed exclusively on a sidewall of the gate electrode trench, as opposed to the substrate trench sidewalls.
2. Description of the Related Art
Vertical sidewall field effect transistors (FETs) have been previously proposed to increase device density in Dynamic Random Access Memory (DRAM) cells. The vertical FET is built along the sidewall of a trench with its source at the bottom of the trench and its drain at the top. Current flows between the source and drain in a direction perpendicular to the plane of the wafer surface and thus the depth of the trench is largely determined by the desired source to drain channel length.
It is also possible to build horizontal sidewall FETs along trench sidewalls. In the horizontal sidewall FET, current flows in the direction parallel or horizontal to the plane of the wafer surface, and thus the trench depth adds to the width of the FET.
The density advantage of horizontal FETs over conventional planar FETs is illustrated by the following example. If horizontal FETs having the same width as the depth of the trench are built within the trenches that are 1.0 .mu.m deep and 0.5 .mu.m wide at a distance of 1.0 .mu.m between trenches, then there are 2.0 .mu.m of device width (a 1.0 .mu.m wide horizontal sidewall FET on each trench sidewall), per 1.0 .mu.m of distance along the wafer surface.
To achieve an equal amount of current drive, a conventional planar device would require 2 .mu.m of wafer surface. Therefore, in the given example, the use of horizontal sidewall FETs offers twice the current drive advantage over conventional planar FETs. In summary, the effective width of FETs can be increased by "corrugating" the silicon surface along the direction of the device width to produce "corrugated" FETs.
One problem associated with horizontal sidewall FETs, however, is how to form gate sidewall spacers to control gate-to-drain spacing in order to prevent high parasitic series resistance (underlap) or high overlap capacitance (excessive overlap). This type of problem is associated with a class of three dimensional problems that are confronted whenever it is required to differentiate between the vertical walls of a trench during a process.
For example, referring to FIGS. 1A and 1B, if it were necessary to form a sidewall spacer 4 only along one wall of a trench as illustrated, one way to achieve this objective would involve the following process. First a film is deposited over the trench using Chemical Vapor Deposition (CVD) and is etched (by for example, reactive ion etching (RIE)) to form a spacer. The spacer would be therefore exist on all walls of the trench. It is possible to remove unwanted spacers by masking and using an isotropic etch. However, such a technique is not self-aligned, resulting in spacers 5 and 6 (see FIG. 2) that will extend along trench walls where they are not desired.
In light of the foregoing, there exists a need for a spacer that can be formed exclusively on a single sidewall of a trench defined by the gate polysilicon to serve as a self-aligned gate sidewall spacer in a corrugated FET.